A varactor diode, commonly referred to as a varactor, is a type of PN junction diode that has a high junction capacitance when reverse biased. The capacitance is variable and is a function of the voltage applied at its terminals. Such devices having a variable, or tunable capacitance, are commonly used in inductive (L) capacitive (C) (LC) resonant circuits that are used as tuning circuits, for impedance matching or as isolation circuits.
One technology for fabricating semiconductor devices is a merged gallium arsenide (GaAs) Heterojunction Bipolar Transistor (HBT)—Field Effect Transistor (FET) technology, where the FET is a specialized device integrated with the HBT having characteristics similar to a depletion-mode (d-mode) Metal Semiconductor Field Effect Transistor (MESFET), where a MESFET is a specialized FET. This integrated technology is commonly referred to as “BiFET,” but alternative nomenclature and integration techniques exist to combine an HBT and a FET on GaAs.
Other semiconductor technologies can also be used to create a FET. One example is complementary metal oxide semiconductor (CMOS) technology which integrates both n-type and p-type e-mode MOSFETs onto the same silicon substrate. Regardless of the technology used to fabricate a FET, when the device is off, the FET can be controlled to exhibit a variable capacitance. Therefore, it would be desirable to have a way of using a FET as a varactor. If the varactor could be implemented such that it had a wide tuning range with continuously tunable characteristics, it could be used in tunable LC circuits, tunable RF matching networks, and any other application requiring an electronically tunable capacitance.